The invention relates generally to electrically conductive polyvinylidene fluoride composites containing carbon nanotubes, and the methods for preparing them.
Polyvinylidene Fluoride
Plastics are synthetic polymers which have a wide range of properties that make them useful for a variety of applications ranging from packaging and building/construction to transportation; consumer and institutional products; furniture and furnishings; adhesives, inks and coatings and others. In general, plastics are valued for their toughness, durability, ease of fabrication into complex shapes and their electrical insulation qualities.
One such widely used plastic is polyvinylidene fluoride (xe2x80x94H2Cxe2x95x90CF2xe2x80x94), (xe2x80x9cPVDFxe2x80x9d), which is the homopolymer of 1,1-difluoroethylene, and is available in molecular weights between 60,000 and 534,000. This structure, which contains alternating xe2x80x94CH2xe2x80x94 and xe2x80x94CF2xe2x80x94 groups along the polymer backbone, gives the PVDF material polarity that contributes to its unusual chemical and insulation properties.
PVDF is a semicrystalline engineered thermoplastic whose benefits include chemical and thermal stability along with melt processibility and selective solubility. PVDF offers low permeability to gases and liquids, low flame and smoke characteristics, abrasion resistance, weathering resistance, as well as resistance to creep and other beneficial characteristics. As a result of its attractive properties, PVDF is a common item of commerce and has a wide variety of applications (e.g., cable jacketing, insulation for wires and in chemical tanks and other equipments).
In addition to forming a homopolymer, PVDF also form co-polymers with other polymer and monomer families, most commonly with the co-monomers hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), and tetrafluoroethylene (TFE), as well as terpolymers and olefins. The properties of the copolymers is strongly dependent on the type and fraction of the co-monomers as well as the method of polymerization. For example, HFP makes a homogenous copolymer with PVDF. On the other hand, the PVDF copolymer phase segregates if the other monomer is not fluorinated.
Conductive Plastics
Recently, demand and applications for electrically conductive plastics have grown. In these uses, one seeks to exploit the unique properties of plastics, often as an alternative to metals. For example, electrically conductive polymeric materials are desirable for many applications including the dissipation of electrostatic charge from electrical parts, electrostatic spray painting and the shielding of electrical components to prevent transmission of electromagnetic waves.
Conductivity (i.e., the ability of material to conduct or transmit heat or electricity) in plastics is typically measured in terms of bulk resistivity (i.e., volume resistivity). Bulk resistivity, which is the inverse of conductivity, is defined as the electrical resistance per unit length of a substance with uniform cross section as measured in ohm-cm. Thus, in this manner, the electrical conductivity of a substance is determined by measuring the electrical resistance of the substance.
Electrically conductive plastics can be divided into several categories according to their use. For example, high level of resistivity (i.e., low level of conductivity) ranging from approximately 104 to 108 ohm/cm generally confer protection against electrostatic discharge (xe2x80x9cESDxe2x80x9d) and is referred to as the ESD shielding level of conductivity. This is also the level of conductivity needed for electrostatic painting. The next level of resistivity, which ranges from approximately 104 ohm/cm and lower, protects components contained within such plastic against electromagnetic interference (xe2x80x9cEMIxe2x80x9d) as well as prevents the emission of interfering radiation, and is referred to as the EMI shielding level of conductivity. In order for a plastic article to be used as a conductive element like a current collector or separator plate in an electrochemical cell, resistivity less than 102 ohm/cm is required.
The primary method of increasing the electrical conductivity of plastics have been to fill them with conductive additives such as metallic powders, metallic fibers, ionic conductive polymers, intrinsically conductive polymeric powder, e.g., polypyrrole, carbon fibers or carbon black. However, each of these approaches has some shortcomings. Metallic fiber and powder enhanced plastics have poor corrosion resistance and insufficient mechanical strength. Further, their density makes high weight loadings necessary. Thus, their use is frequently impractical.
When polyacrylonitrile (xe2x80x9cPANxe2x80x9d) or pitch-based carbon fiber is added to create conductive polymers, the high filler content necessary to achieve conductivity results in the deterioration of the characteristics specific to the original resin. If a final product with a complicated shape is formed by injection molding, uneven filler distribution and fiber orientation tends to occur due to the relatively large size of the fibers, which results in non-uniform electrical conductivity.
Principally because of these factors and cost, carbon black has become the additive of choice for many applications. The use of carbon black, however, also has a number of significant drawbacks. First, the quantities of carbon black needed to achieve electrical conductivity in the polymer or plastic are relatively high, i.e. 10-60%. These relatively high loadings lead to degradation in the mechanical properties of the polymers. Specifically, low temperature impact resistance (i.e., a measure of toughness) is often compromised, especially in thermoplastics. Barrier properties also suffer. Sloughing of carbon from the surface of the materials is often experienced. This is particularly undesirable in many electronic applications. Similarly, outgassing during heating may be observed. This adversely affects the surface finish. Even in the absence of outgassing, high loadings of carbon black may render the surface of conductive plastic parts unsuitable for automotive use.
Taken as a whole, these drawbacks limit carbon black filled conductive polymers to the low end of the conductivity spectrum. For EMI shielding or higher levels of conductivity, the designer generally resorts to metallic fillers with all their attendant shortcomings or to metal construction or even machined graphite.
What ultimately limits the amount of carbon black that can be put into plastic is the ability to form the part for which the plastic is desired for. Depending on the plastic, the carbon black, and the specific part for which the plastic is being made, it becomes impossible to form a plastic article with 20-60 wt % carbon black, even if the physical properties are not critical.
Carbon Fibrils
Carbon fibrils have been used in place of carbon black in a number of polymer applications. Carbon fibrils, referred to alternatively as nanotubes, whiskers, buckytubes, etc., are vermicular carbon deposits having diameters less than 1.0xcexc and usually less than 0.2xcexc. They exist in a variety of forms and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces. Such fibers provide significant surface area when incorporated into a structure because of their size and shape. They can be made with high purity and uniformity.
It has been recognized that the addition of carbon fibrils to polymers in quantities less than that of carbon black can be used to produce conductive end products. For example, U.S. Pat. No. 5,445,327, hereby incorporated by reference, to Creehan disclosed a process for preparing composites by introducing matrix material, such as thermoplastic resins, and one or more fillers, such as carbon fibers or carbon fibrils, into a stirred ball mill. Additionally, U.S. Ser. No. 08/420,330, entitled xe2x80x9cFibril-Filled Elastomer Compositions,xe2x80x9d also incorporated by reference, disclosed composites comprising carbon fibrils and an elastomeric matrix, and methods of preparing such.
It has also been recognized that the addition of carbon fibrils to polymers can be used to enhance the tensile and flexural characteristics of end products. (See, e.g. Goto et al., U.S. application Ser. No. 511,780, filed Apr. 18, 1990, and hereby incorporated by reference.)
Additionally, prior work by Moy et al., U.S. application Ser. No. 855,122, filed Mar. 18, 1992, and Uehara et al., U.S. application Ser. No. 654,507, filed Feb. 23, 1991, both incorporated by reference, disclosed the production of fibril aggregates and their usage in creating conductive polymers. Moy et al. disclosed the production of a specific type of carbon fibril aggregate, i.e. combed yarn, and alluded to its use in composites. Uehara et al. also disclosed the use of fibril aggregates in polymeric materials. The fibril aggregates have a preferred diameter range of 100-250 microns. When these fibril aggregates are added to polymeric compositions and processed, conductivity is achieved.
U.S. Pat. No. 5,643,502 to Nahass et al., hereby incorporated by reference, disclosed that a polymeric composition comprising a polymeric binder and 0.25-50 weight % carbon fibrils had significantly increased IZOD notched impact strength (i.e., greater than about 2 ft-lbs./in) and decreased volume resistivity (i.e., less than about 1xc3x971011 ohm-cm). Nahass disclosed a long list of polymers (including polyvinylidene fluoride) into which carbon fibrils may be dispersed to form a composite. The polymers used by Nahass in the Examples of the ""502 patent for preparing conductive, high toughness polymeric compositions include polyamide, polycarbonate, acrylonitrile-butadiene-styrene, poly (phenylene ether), and thermoplastic urethane resins and blends.
While the nanotube-containing polymer composites of the art are useful and have valuable strength and conductivity properties, many new uses for such composites require that very high strength and low conductivity be achieved with low nanotube loading in the polymer. Accordingly, the art has sought new composite compositions which achieve these ends.
It is a primary object of the invention to provide a polymer composite which is mechanically strong and electrically conductive.
It is a particular object of the invention to provide a polymer composite which has a higher level of conductivity than known polymer composites.
It is yet another object of the invention to provide polymer composites which achieve extraordinary levels of conductivity at low levels of nanotube loading.
It is a further object of the invention to provide methods for preparing a polymer composite which is mechanically strong and electrically conductive.
It has now been discovered that composites containing polyvinylidene fluoride polymer or copolymer and carbon nanotubes have extraordinary electrical conductivity. Composites with less than 1% by weight of carbon nanotubes have been found to have a bulk resistivity many times lower than the bulk resistivity of other polymer composites having similar nanotube loading. Composites with as little as 13% by weight carbon nanotubes have a bulk resistivity similar to that of pure carbon nanotube mats.
Composites containing polyvinylidene fluoride polymer or copolymer and carbon nanotubes may be prepared by dissolving the polymer in a solvent to form a polymer solution and then adding the carbon nanotubes into the solution. The solution is mixed using a sonicator or a Waring blender. A precipitating component is added to precipitate out a composite comprising the polymer and the nanotubes. The composite is isolated by filtering the solution and drying the composite.
Composites containing polyvinylidene fluoride polymer or copolymer and carbon nanotubes may also be prepared by adding carbon nanotubes to a polymer emulsion and then mixing the emulsion with a Waring Blender. The water/solvent is then removed by evaporation to obtain the composite comprising the polymer and the nanotubes.